Princeton University researchers are throwing some cold water on the hot notion that astrobiologists and other scientists expect to one day find life on other planets.

Recent discoveries of planets similar to Earth in size and proximity to the planets' respective suns have sparked scientific and public excitement about the possibility of also finding Earth-like life on those worlds, but the expectation that life -- from bacteria to sentient beings -- has or will develop on other planets as on Earth might be based more on optimism than scientific evidence stated Princeton astrophysical sciences professor Edwin Turner and David Spiegel, a former Princeton postdoctoral researcher and now with the Institute for Advanced Study in a paper on the subject.

In fact the current knowledge about life on other planets suggests that it's very possible that Earth is a cosmic aberration where life took shape unusually fast. If so, then the chances of the average terrestrial planet hosting life would be low, Turner and Spiegel state. Finding a single case of life arising independently of our lineage (on Earth, elsewhere in the solar system, or on an extrasolar planet) would provide much stronger evidence that the emergence of life or what the researchers call abiogenesis is not extremely rare in the universe.

According to a release, the researchers used a Bayesian analysis -- which weighs how much of a scientific conclusion stems from actual data and how much comes from the prior assumptions of the scientist -- to determine the probability of extraterrestrial life once the influence of these presumptions is minimized.

"The idea that life has or could arise in an Earth-like environment has only a small amount of supporting evidence, most of it extrapolated from what is known about abiogenesis on early Earth. Instead, their analysis showed that the expectations of life cropping up on exoplanets -- those found outside Earth's solar system -- are largely based on the assumption that it would or will happen under the same conditions that allowed life to flourish on this planet," the researchers said.

"Fossil evidence suggests that life began very early in Earth's history and that has led people to determine that life might be quite common in the universe because it happened so quickly here, but the knowledge about life on Earth simply doesn't reveal much about the actual probability of life on other planets," Turner said. "Information about that probability comes largely from the assumptions scientists have going in, and some of the most optimistic conclusions have been based almost entirely on those assumptions."

"If scientists start out assuming that the chances of life existing on another planet as it does on Earth are large, then their results will be presented in a way that supports that likelihood," Turner said. "Our work is not a judgment, but an analysis of existing data that suggests the debate about the existence of life on other planets is framed largely by the prior assumptions of the participants."

The idea that we may someday sooner rather than later spot extraterrestrial life is deeply enhanced by some of the spectacular findings by NASA's Kepler plant-finding satellite telescope and other space projects like the Probing Lensing Anomalies NETwork (PLANET) study this year that said our Milky Way galaxy holds at least 100 billion planets -- a minimum of one planet for every star on average. This finding means that there could be a minimum of 1,500 planets within 50 light-years of Earth.

In December 2011, NASA announced the first observation of Kepler-22b, a planet 600 light years from Earth and the first found within the habitable zone of a Sun-like star. Weeks later, NASA reported Keplers-20e and -20f, the first Earth-sized planets found orbiting a Sun-like star. In April 2012, NASA astronomers predicted that the success of Kepler could mean that an "alien Earth" could be found by 2014 -- and on it could dwell similar life, the researchers noted.

While these observations tend to stoke the expectation of finding Earth-like life, they do not actually provide evidence that it does or does not exist, instead, these planets have our knowledge of life on Earth projected onto them, Spiegel said.